(1) Field of the Invention
This invention relates to the electrode structure and geometry of ion traps in general and to quadrupole ion traps and associated mass spectrometers in particular.
(2) Description of the Related Art
The ion trap of an ion trap mass spectrometer, in its most common configuration, is composed of a central ring electrode and two end cap electrodes (end caps). Generally, in longitudinal section, each electrode has a convex surface facing an internal volume known as the trapping volume. These surfaces are typically defined by central segments of a polynomial, which are often largely hyperbolic with small components of additional terms. In addition to providing a trapping space for ions, the trapping volume also serves as an analyzing space in which selected ions are retained and sequentially ejected, based upon their mass and charge (mass-to-charge ratio or m/z). It also serves as a reaction volume, in which fragmentation of charged particles is caused both by collisions and by interactions with additional specific fields. When a radio frequency (RF) voltage is applied between the ring and end cap electrodes, an electric potential is induced within the trapping volume which varies quadratically with displacement from the center of the trap. This potential produces a linear electric field which is advantageous for control of ion motion. Ions introduced into or formed within the trapping volume will or will not have stable trajectories, depending upon their mass, charge, the magnitude and frequency of the applied voltages, and the dimensions and geometry of the three electrodes.
Quadrupole ion trap potentials, and thus fields, deviate from the ideal for several reasons: 1) because the electrodes are of finite size; 2) because the shape or position of the electrodes are non-ideal; and 3) because of the apertures added to the end caps for introducing ions or electrons into the trapping volume and for ejecting ions from the trapping volume to an external detector. These deviations are referred to as field faults.
In the context of mass spectrometry using quadrupole ion traps, the field faults can result in both peak broadening and, in some cases, a shift in the measured ion mass from the theoretical mass values. Several techniques have been used and proposed to neutralize field fault effects on the motion of the trapped ions. See, for example, Franzen et al. U.S. Pat. No. 5,468,958, which describes a quadrupole ion trap with switchable multipole fractions which can be used to correct the electric potential errors due to the finite size of the electrodes, and Franzen et al. U.S. Pat. No. 6,297,500, which describes an electrode structure in which these electric potential errors due to the finite size of the electrodes is proposed to be corrected by narrowing the gap width between the ring and end cap electrodes at the edge regions where these electrodes are most closely proximate.
The field faults caused by the apertures in the end caps are generally more significant than those caused by finite electrode size. One method for correcting the deviations due to the apertures is to stretch the distance (z0) between the end cap electrodes, and thus the spacing of one or both of the end cap electrodes from the ring electrode, beyond the theoretical spacing predicted by solving the equations of motion of charged particles contained within the trapping volume. Another approach is found in Kawato, U.S. Pat. No. 6,087,658, in which the inner surface of each end cap electrode is modified by the addition, around at least one of the apertures thereof, of a bulge protruding from the hyperbolic surface and extending inward to the associated aperture. The bulge is asserted to control the deviation in the electric potential around the end cap apertures from the ideal quadrupole electric potential.
The use of such altered electrode geometries provides a first order correction of field faults caused by the apertures, and an overall improvement in the linearity of the field. However, the overall improvement in the field linearity with the prior art methods can not be obtained without an unintentional degradation of the field in localized areas (e.g., at key locations between the trap center and the apertures in the vicinity of 60-70% of the distance therebetween).
Non-hyperbolic electrodes have been studied and implemented for quadrupole ion traps so as to take advantage of the material and labor economies associated with manufacturing electrodes of simpler shapes, such as cylindrical or spherical, but typically provide performance that is inferior to standard hyperbolic electrodes (Wells, et al., “A Quadrupole Ion Trap with Cylindrical Geometry Operated in the Mass-Selective Instability Mode” Analytical Chemistry, 70, 438-444, 1998).